Horse Parasite by sciurus
Once infected, patches of the horse's hair begin to fuse into a hard, chitinous material and the back of its skull elongates, eventually forming a second head. As the parasite grows, it gains greater control over its host's body and the horse's intestines are hastily adapted to the parasite's carnivorous diet. The newly rearranged stomach is inefficient and the creature gradually withers away, although not before the parasite has had the opportunity to release its eggs.
The creature was outlined in pen then coloured in Photoshop. The background is a section of some random ink patterns.